In recent years, with the advance of environmental technology, development of power generation devices (e.g., solar power generation devices) which pose less burden on the environment than conventional power generation methods has been actively conducted. Concurrently with the development of power generation technology, development of power storage device has also been underway.
As an example of a power storage device, a lithium ion battery which is a secondary battery can be given. A lithium ion battery is widely prevalent since energy density thereof is high and the lithium ion battery is well suited for miniaturization. As an electrode material of a lithium ion battery, a material in/from which lithium ions can be inserted and extracted is preferably used. Examples thereof include graphite and silicon. Silicon is particular expected to be a host material of lithium because theoretical capacity of silicon is ten times as high as that of graphite.
However, there is a problem in a secondary battery whose electrode includes silicon or a silicon compound, in that charge-discharge cycle characteristics and storage characteristics are not sufficient when LiPF6, LiBF4, LiAsF6, or the like is used as an electrolyte. In Patent Document 1, is disclosed a secondary battery in which charge-discharge cycle characteristics and storage characteristics are sufficiently obtained by setting the concentration of at least one kind of those electrolytes to lower than 0.1 mol·dm−3.